BC can be classified into Luminal A (Estrogen Receptor (ER)+ or Progesterone Receptor (PR)+, and Human Epidermal growth factor Receptor 2 (HER2)−), Luminal B (ER+ or PR+, HER2+), HER2-positve (ER− and PR−, HER2+), and Triple Negative (ER− and PR−, HER2−) subtypes. This evidence concerns the gene ESR1 and breast cancer.